Photovoltaic devices, particularly photovoltaic devices manufactured from deposited layers of thin film semiconductor material, preferably include a back reflector disposed beneath the photoactive semiconductor layers. The back reflector redirects light which has passed through the photovoltaic body unabsorbed, back through that body for further absorption. The use of a back reflector increases the efficiency of the photovoltaic device and permits the use of relatively thinner layers of photoactive material, thereby enhancing the collection efficiency of the device as well as producing a savings in materials. In many instances, the back reflectors are textured so as to provide a diffuse reflecting surface which increases the path length of the reflected light and also provides for enhanced internal reflection.
It is fundamental that a back reflector be highly reflective of light having a wavelength corresponding to the photo response spectrum of the semiconductor layers. In general, it has been found that metals such as aluminum, silver, and copper are good reflectors for thin film photovoltaic devices, such as silicon alloy devices. It is also most important that the materials comprising the back reflector not react with one another or with other materials in the photovoltaic device, either during its manufacture or its use. Such reactions can degrade the efficiency of the device or render it inoperative. Back reflector structures must also be mechanically compatible with the remaining layers of the photovoltaic device. The thinness of the layers can make the device sensitive to a number of mechanical defects. Spikes, or other protrusions in the substrate or in the back reflector can produce shunts or short circuits through the semiconductor layers, thereby compromising device function. Additionally, a back reflector structure which cracks, deforms or otherwise delaminates from the substrate can destroy superjacent semiconductor layers to the detriment of device function. Accordingly, it will be appreciated that back reflectors for thin film photovoltaic devices should be highly reflective of light, nonreactive with adjacent layers of the device, and mechanically compatible with device structure and processing.
The simplest prior art approach to provide a back reflector comprises depositing the various layers of the photovoltaic device upon a highly polished substrate, typically stainless steel. However, while stainless steel is mechanically stable and nonreactive, it is not very highly reflective of light; the integrated reflectivity of most stainless steels is only about 45%. Furthermore, providing a textured reflective surface to the stainless steel is somewhat difficult. As a consequence, more sophisticated back reflector structures have been developed. Aluminum, with or without a silicon alloying agent, is highly reflective of light and the texture of the layer may be controlled via the parameters of the deposition process. Aluminum is somewhat reactive and as a consequence, in most instances it is coated with a protective layer of a conductive material such as a metallic oxide which may optionally include an additional metal. In some instances, the aluminum is deposited as a specular; i.e., smooth, reflector and a textured protective layer is deposited thereatop to provide for light scattering. Back reflectors of this type are presently incorporated in a variety of photovoltaic devices and are disclosed, for example, in U.S. Pat. No. 5,101,260.
While aluminum is a good light reflector, silver and copper are better yet, particularly for those portions of the electromagnetic spectrum which correlate to the response spectrum of thin film silicon alloy materials; and as a consequence, it is highly desirable to include these metals in back reflective structures. Silver/zinc oxide back reflectors have been employed in the past and U.S. Pat. No. 4,816,082 describes a photovoltaic device having a back reflector of this type. Silver is a relatively soft metal and it has been found that while high efficiency photovoltaic devices can be manufactured with a silver/zinc oxide back reflector, significant problems of yield occur which limit the commercialization of this type of device. These problems are particularly severe when the silver layer is relatively thick, as for example when it is texturized, and it is speculated that the soft silver deforms during processing and use of the photovoltaic device thereby giving rise to a variety of failures. In an attempt to overcome this problem, a conventional aluminum silicon alloy back reflector was provided with a relatively thin layer of silver thereupon. This technology is described in the aforementioned U.S. Pat. No. 5,101,260. Again, it was found that while the silver enhanced the efficiency of the device somewhat, the efficiency was not as good as that attained using a thick silver/zinc oxide reflector. It is the speculation of the present inventors that reaction between the silver and aluminum layers during the manufacture and/or use of the device results in a lowered reflectivity of the metal which lowers the short circuit current density, and hence the efficiency, of the device.
The present invention, as will be described in greater detail hereinbelow, provides a highly reflective back reflector structure which may optionally be texturized. Photovoltaic devices including the back reflector of the present invention may be readily manufactured in high yield and exhibit high efficiency and stability in use. These and other advantages of the present invention will be readily apparent from the drawings, discussion and description which follow.